Enhanced X-Analytics header with HTTPS and Proxy information

* Added X-Forwarded-Proto and X-Forwarded-By values to X-Analytics hdr
* Moved X-Analytics assembly to the vcl_deliver since some values
may not be varyied on, and need to be combined with the backend response
* X-Analytics uses variables (headers) that are local to different files,
so now each file handles just the variables it knows about, thus reducing
cross-file dependency/knowledge.
* Moved most mentionings of X-CS to zero.vcl, since that's the only file
that really knows about them.

Change-Id: I4fe500ce34973df63fb67dc22b0ffa1b3d3b0557